Study Of The Effect Between Rapid Heat Treatment In The Laboratory And Intermediate Test On The Microstructure And Mechanical Properties Of Automotive Martensite Steel

With the rapid development of heat treatment technology,the exploitation and application of rapid heat treatment is getting more widely noted and studied.It bases on the core of repid heating,isothermal treatment for short time and rapid cooling,which Provides valuable academic support and practical experiences to research commercial steel,especially high strength steel used to the car.Advanced High-Strength Steel which combine high strength with good plasticity can be abtained through such technology,and meet the need of weight loss and safety for automobile in the automobile industry.The development of industry aggravates the consumption of the energy and damage of the environment,therefore,the research of Advanced High Strength Steel has a profound influence on the automobile industry which develops to right direction of safety,saving and low-carbon.The low-carbon martensitic steel is one of the Advanced High Strength Steels,which are widely used to auto parts,such as side door beams and so on.Over the past ten years,the study about microstructure and properties of low-carbon martensite steel made by rapid heat treatment has no better results,so it need futher stuy.In this paper,the low carbon steel B800NQ made by some company in China as the research object,on the basis of previous studies,The curves of CHT and CCT about B800NQ are determined by automatic Phase change instrument,meanwhile,Ferrite recrystallization and Austenite transition of B800NQ are studied through annealing at salt bath furnace.These datas and results lay the foundation for future experiments;Simulating the rapid heat treatment experiment through salt bath furnace and beltline made in Bao Steel company,to study on the mechanical properties and microstructure of experimental steel;analyzing the microstructure and mechanical properties with OM,SEM,TEM and Electronic Material Testing Machine.The research results turn out as followed:Firstly,experimental results of heating treatment in the Lab include that ferrite recrystallization temperature of cold rolled B800NQ is 695 ?,finished temperature at 720?740?;ferrite recrystallization occurs before austenite transition,austenite transition temperature at 720?,the formation of austenite mainly prefers to occur in the pearlite area,later appears at ferrite boundary.Secondly,simulating rapid heat treatment at salt bath furnace acquires martensite steels which own Satisfactory microstructure and mechanical properties,the heating temperature is 916? or 930? keeping 30s and which is 930? or 960? holding 20s enhance the tensile strength to 1400MPa,elongation is more than 6.8%(reaching expected target),the microstructure is almost hard martensite phase;the prior austenite grain size has obvious influence on the performance.The two process such as 916? holding 30s and 930? keeping 20s are better,the tensile strength are more than 1450MPa and elongation are also reaching7.2%,the average grain size of prior austenite is only about 5?m,fine-grain strengthening and dislocation strengthening are obvious;with the heating temperature increasing to 960?,the grain size is close to 9?m and austenite grows up abnormally,so the tensile strength and elongation becoming worse;the lath martensite has a multi-level structure including prior austenite grain,packet,block and lath structure,the packet and block are both have high angle boundaries;Once the prior austenite grain size is less than 12?m,most grains are composed of only one packet(packet contains one block).Finally,simulating rapid heat treatment at intermediate test in Bao Steel company.With 10?/s,80?/s,200?/s and 300?/s heating rate,holding 5s,10s and 20s,the results are unsatisfactory.The process which heat as 300?/s and keep 20s could make tensile strength to 1088 MPa,however,others are about 1030MPa,much lower than salt bath furnace processes;compared with rapid heat treatment with salt bath furnace in the laboratory,deviations of temperature-controlling,unevenly heating,low cooling rate and incompletely austenitizing lead to the microstructure including more soft ferrite so that aggravateing tensile strength.